The art of flash-spinning plexifilamentary film-fibrils from a polymer in a solution or a dispersion is known in the art. The term "plexifilamentary" means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean thickness of less than about 4 microns and with a median fibril width of less than about 25 microns. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form the three-dimensional network.
The process of forming plexifilamentary film-fibril strands and forming the same into non-woven sheet material has been disclosed and extensively discussed in U.S. Pat. No. 3,081,519 to Blades et al.; U.S. Pat. No. 3,227,794 to Anderson et al.; U.S. Pat. No. 3,169,899 to Steuber; and U.S. Pat. No. 3,860,369 to Brethauer et al. (all of which are assigned to E. I. du Pont de Nemours and Company ("DuPont")). This process and various improvements thereof have been practiced by DuPont for a number of years in the manufacture its Tyvek.RTM. spunbonded olefin.
The general flash-spinning apparatus shown in FIG. 1 is similar to that disclosed in U.S. Pat. No. 3,860,369 to Brethauer et al., which is hereby incorporated by reference. According to the flash-spinning process, a mixture of polymer and spin agent is provided through a pressurized supply conduit 13 to a spinning orifice 14. The polymer mixture in chamber 16 is discharged through a spin orifice 14 where extensional flow near the approach of the orifice helps to orient the polymer into elongated polymer molecules. When polymer and spin agent discharge from the orifice, the spin agent rapidly expands as a gas and leaves behind fibrillated plexifilamentary film-fibrils. The spin agent's expansion during flashing accelerates the polymer so as to further stretch the polymer molecules just as the film-fibrils are being formed and the polymer is being cooled by the adiabatic expansion. The quenching of the polymer freezes the linear orientation of the polymer molecule chains in place, which contributes to the strength of the resulting flash-spun plexifilamentary polymer structure.
The polymer strand 20 discharged from the spin orifice 14 is directed against a rotating lobed deflector baffle 26 that spreads the strand 20 into a more planar web structure 24, and alternately directs the web to the left and right as the web descends to a moving collection belt 32. The web forms a fibrous batt 34 that is passed under a roller 31 that compresses the batt into a sheet 35 formed with plexifilamentary film-fibril networks oriented in an overlapping multi-directional configuration. The sheet 35 exits the spin chamber 10 through the outlet 12 before being collected on a sheet collection roll 29. The sheet 35 may be thermally bonded in order to obtain desired sheet strength, opacity, moisture permeability and air permeability.
The polymers that have been conventionally used in production of flash-spun plexifilamentary sheets are polyolefins, especially polyethylene. British Patent Specification 891,943 (assigned to DuPont) discloses that additives, including colored pigments, can be added to the polymeric material used in producing flash-spun plexifilamentary fibers. U.S. Pat. No. 3,169,899 (assigned to DuPont) suggests that flash-spun polymer with various additives, including pigments, may be used in producing plexifilamentary sheet material. However, this prior art does not disclose or suggest how pigments might be used to produce sheet material with improved physical properties or what the properties of such sheet material might be.
It has been found that the delamination strength of a flash-spun polyethylene sheet of a given basis weight can be significantly increased by increasing the amount of thermal bonding to which the sheet is subjected. However, the opacity of flash-spun plexifilamentary sheets decreases with increased amounts of thermal bonding. Reduced opacity gives many highly bonded sheets a flimsy and mottled appearance, even though such sheets may actually have a higher strength than less bonded sheets. Reduced opacity may also cause quicker degradation of sheet strength in the presence of ultraviolet light, such as sunlight, because more light passes through a less opaque sheet. In addition, when a less opaque sheet is printed, the printed matter is much more difficult to read than printed matter on a sheet with higher opacity. The traditional tradeoff between delamination strength and sheet appearance has been troublesome in a number of the end use applications for flash-spun sheet material, including sterile packaging, maps and envelopes.
When used as a sterile packaging material, flash-spun sheet material is made into packaging for items that require sterilization, such as surgical instruments. An item is placed in a pouch or other package made of flash-spun sheet material, which package is then sealed and sterilized. The package seal is subsequently opened to remove the sterilized item. When the sterilized item is something like a surgical instrument, it is extremely important that the sheet not tear or delaminate when opened because this would generate particulates that could deposit on the instruments. Resistance to delamination can be increased by increasing the amount of bonding to which the sheet is subjected. However, when a lower basis weight sheet material is heavily bonded, the sheet takes on a translucent and mottled appearance that makes users question the sterility of items stored in such material. In the past, sheets with basis weights higher than what is needed for strength and bacterial barrier properties have been used in sterile packaging in order to provide a desired level of opacity. A flash-spun sheet material is needed that can be used at lower basis weights than the sheet material currently used in sterile packaging, yet can be thermally bonded to the degree necessary to obtain the requisite delamination strength without taking on an unacceptable translucent and mottled appearance.
Another end use in which high opacity, good visual uniformity and high delamination strength of a bonded flash-spun plexifilamentary sheet offers great advantages is for printed materials, such as maps and tags. Certain maps, such as marine maps and military maps, need to be durable under a variety of adverse conditions. Maps printed on bonded flash-spun sheet material have been found to be offer such durability. Because the users of such maps frequently plot courses on the maps and later erase the course markings, the maps must resist abrasion-induced delamination and scuffing of the surface. This abrasion resistance is best achieved by increasing the degree of sheet bonding. In addition, flash-spun plexifilamentary sheet material can be more readily printed if it has a smooth surface. A bonded plexifilamentary sheet material can be made smoother by passing the sheet between smooth thermal calender rolls. At the same time, high sheet opacity is needed if detailed printing is to be readable from the sheet on which a map is printed. Unfortunately, sheet opacity is normally reduced when a sheet is subjected to higher levels of bonding and/or to thermal calendering. In the past, the basis weight of plexifilamentary sheet material has been increased in order to meet the printing requirements of high sheet opacity, high delamination strength, and high sheet smoothness. However, heavier sheet material also makes for printed sheets that are heavier, bulkier and less flexible than is desirable.
Accordingly, there is a need for a plexifilamentary sheet that can be subjected to substantial thermal bonding and/or thermal calendering without undergoing a significant reduction in the opacity of the sheet. There is also a need for a sheet material that when printed is highly readable, even by bar code scanning equipment. Finally, there is a need for opaque plexifilamentary sheets that are colored and that exhibit a high degree of color saturation after thermal bonding.